The simultaneous resistance of cancer cells to many different anti-cancer drugs is the major impediment to successful chemotherapy of metastatic disease. An important mechanism of multidrug resistance is expression of P-glycoprotein, a 170,000 dalton energy-dependent drug efflux pump which removes natural product drugs from the cell. We have continued our studies of multidrug resistance by an analysis of the mechanism by which this pump removes drug from within the plasma membrane or from the cytoplasm. In an in vitro vesicle system, ATP has been shown to be the preferred energy source, and many of the drugs which are transported compete with each other for a single site or small number of sites on the transporter. Labeling sites for the P-glycoprotein inhibitor 3H-azidopine occur in both the amino and carboxy-terminus of the protein, and these two sites appear likely to make up the single channel through which the drugs move. Molecular manipulations have identified the first intracytoplasmic loop as a domain involved in drug recognition, which is distinct from the drug labeling sites identified with 3H-azidopine. We have also developed an MDR1 transgenic mouse whose bone marrow is protected from the cytotoxic effects of anti-cancer drugs by expression of P-glycoprotein. This model can be used to identify potent agents which inhibit the multidrug transporter in vivo, since these agents sensitize the transgenic mice to the leukopenia induced by chemotherapy. The MDR 1 cDNA can also be introduced into mouse bone marrow by retroviral infection. Such MDR1 retroviral vectors should be useful for gene therapy to protect bone marrow during cancer therapy and to introduce non-selectable genes into bone marrow. New in vitro models of resistance to VP-16 and cis-platinum, not involving the multidrug transporter, are under development.